STORY ARCHIVE

Drought has gripped south-west Australia for decades, and while climate scientists blame a mix of natural weather cycles and human activity, the precise cause remains open to debate. With 750 years of climate data revealed in ice cores from Law Dome in Antarctica, Mark Horstman reports on the discovery of a link between snowfall there, and decreases in WA's rainfall.

Ice Core Drought

TRANSCRIPT

Reporter in 1971What’s gone wrong, what’s happened and in fact, will it ever rain again?

NARRATIONThe settlement of Western Australia is built on reliable rainfall.

Scientist in 1970sI think it is practically certain that we will get more rain and it will come of course in winter as usual.

NARRATIONBut that wasn’t to be...

Mark HorstmanSince the 1970s, the south-western corner of Western Australia has suffered a dramatic decline in their winter rainfall, so rapid and so extreme that it’s like, somewhere, a giant tap is being turned off.

Farmer in WAIt’s heartbreaking, it really is. We had some dry years in the '70s but nothing like this.

NARRATIONAnnual rainfall has dropped by 15 per cent since then, most sharply in the winter months. The last place one would expect to find a clue to the mystery of the vanishing rain, is at Law Dome, in the depths of an Antarctic blizzard.

Dr Tas van OmmenIt was rather a strange experience being there at Law Dome and realising that the snow that was burying us was actually telling us something about climate as far away as Australia. Ice cores are arguably the best record that we have on past climate because of the huge range of things that they actually archive within the ice. Water in the ice itself even tells us about past temperatures.

NARRATIONThe ice cores from Law Dome show unusually high snowfall over the last 30 years the same period of time south-west WA has been crippled by drought.

Dr Tas van OmmenWhat we found when we looked at the pattern responsible for the snowfall in Antarctica was this curious connection with dry air coming to Western Australia.

NARRATIONOne of the favourite theories to explain the drought has been changes in the giant vortex of high-speed winds that whip around the Southern Ocean. It’s called the Southern Annular Mode, where low-pressure systems whirling northward lick the southern fringes of Australia, bringing rain. In recent decades, the polar vortex has contracted and these storm tracks have been missing the land more often.

Dr Tas van OmmenBut there’s also a puzzle in this because there’s not been a lot of change in the Southern Annular Mode in winter, and yet the West Australian drought is a winter phenomenon. So that told us for some time that there was a missing piece of the jigsaw puzzle.

Mark HorstmanThe answer to the mystery may lie in this cold room at the University of Tasmania, at 20 degrees below zero.

NARRATIONHere Tas and his team carefully analyse the trace elements trapped inside an ice core of compressed snow that stretches back 750 years.

Mark HorstmanCan you see bands in the ice like you see rings in a tree?

Dr Tas van OmmenNormally you can’t see anything by eye in these ice cores, but by the time you make chemical measurements down the ice you can actually see the layers and the seasonal cycles.

Mark HorstmanSo it’s like a rain gauge, if you like, but deep frozen?

Dr Tas van OmmenYes, exactly. When we look at the 750 years of snowfall record from Law Dome, we see that the last 30 years really stand out quite distinctly. The 750 years gives us an envelope of what’s natural variation at Law Dome and the last 30 years has got snowfall that’s really significantly higher.

NARRATIONBut more than that it provides an unexpected link with the Big Dry in WA.

Dr Tas van OmmenWe found an inverse relationship between snowfall at Law Dome and the rainfall in Western Australia

NARRATIONThat means more snow in East Antarctica is linked to less rain in Western Australia. And that leads to a new explanation for the drought.

Dr Tas van OmmenThere’s a natural circulation that’s driven by three ocean basins and the three continents and it’s called the Wave Three circulation.

NARRATIONWave Three because it creates three pairs of high and low pressure centres that deliver warm moist air to Antarctica while pushing cool dry air over Australia.

Dr Tas van OmmenSo when there’s extra moisture at Law Dome, the same circulation pattern is starving Western Australia of moisture.

NARRATIONThis circulation pattern has intensified over the last 30 years.

Dr Tas van OmmenNow it could be just a very rare event, but we tend to think given that it coincides with human induced changes on the atmosphere and given that modelling evidence suggests we might see a strengthening of the zonal Wave Three, put all these things together and it provides quite strong evidence that ozone plays an important role, CO2 probably does as well. It’s statistically a really significant event. It looks like the sort of event you would see in five, 10, 30,000 years, a very long time, but of course until we get a longer record we can’t pin that down as tightly as we’d like.

NARRATIONThe analysis of older ice cores continues... as does the Big Dry in Western Australia.

Dr Tas van OmmenThe last 30 years is not natural. By doing this very practical climate science we’re actually understanding things that are going to help farmers plan for years to come... and if you can adapt to change then you’re really going to be ahead of the curve.